1. Field of the Invention
The present invention relates to a photodetector. The present invention more specifically relates to a photodetector comprising a semiconductor layer having a thickness smaller than the wavelength of the light to be detected.
2. Discussion of the Related Art
Current photodetectors generally comprise a semiconductor layer associated with a charge transfer device. When a light beam illuminates the semiconductor layer, the incident photons form electron-hole pairs therein. The carriers are then transferred by the charge transfer device to an electronic circuit which enables them to be quantified.
The average depth at which electron-hole pairs are formed in a semiconductor layer depends on the wavelength of the incident light beam and on the semiconductor material used: the greater the wavelength, the deeper the electron-hole pairs are likely to form in a semiconductor layer. For example, to absorb approximately 99% of red light, it is necessary to have a silicon layer with a thickness of approximately 10 μm. For blue light, a silicon layer having a thickness on the order of 3 μm is sufficient to reach such an absorption rate. Such thicknesses are much greater than the wavelengths of the light waves which are desired to be detected and become prohibitive in the case of far infrared or of terahertz waves.
In the case of an image detector, the size of the pixels of elementary photodetectors is generally desired to be decreased. The decrease in the surface area taken up by each pixel on the semiconductor layer especially poses problems of isolation of the photosensitive regions from one another. Indeed, to avoid interferences between neighboring pixels, it is generally provided to form isolating trenches around each of them. If the semiconductor layer is relatively thick, the isolating tranches take up a non-negligible surface area of the semiconductor layer, which is not compatible with the general pixel size decrease. The thickness of the semiconductor layers thus has to be decreased. However, as indicated previously, the forming of very thin active layers does not normally enable to absorb and detect all the incident light.
It has thus been provided to increase the equivalent thickness of the semiconductor photodetector layer by having the photons cross this layer twice or several times. Thus, a mirror may be provided on one side of the semiconductor layer to provide a double travel therein. Reflective elements may also be provided on either side of the semiconductor layer to form structures of Fabry-Pérot type. In practice, the reflective elements are Bragg mirrors which are generally thick, for example formed of a stack of from 10 to 30 quarter-wavelength layers. The advantage of the thickness decrease of the active semiconductor layer is then lost.
To decrease the thickness of the semiconductor photodetector layers, it has been provided to take advantage of physical phenomena of collective electron charge oscillation (plasmon resonance). However, the use of the plasmon resonance generally implies the provision of periodic structures, which makes the detector very selective in terms of wavelength and of angle of incidence. Further, structures using the plasmon resonance must be sized in highly accurate fashion, which makes the manufacturing difficult.